Evidence of Sr mobility in and around the albite-lepidolite-topaz granite of Beauvoir (France): an in-situ ion and electron probe study of secondary Sr-rich phosphates

摘要

The Beauvoir albite-lepidolite-topaz granite is a small, highly evolved intrusion, rich in Li, Rb, F, but also paradoxically rich (or enriched) in Sr (up to 755 ppm) and Ca, a provocative chemical contradiction. Goyazite, an Sr-rich phosphate, is ubiquitous as small secondary grains growing exclusively in albite. Ion microprobe spot analyses reveal variable, higher than whole-rock ~(87)Sr/~(86)Sr ratios at the crystal scale. These data suggest exchanges between the granite and the surrounding micaschists. A two-stage scenario is proposed: (1) Sr, mainly common, was introduced from the micaschists into the Sr-poor granite at the vanishing time of crystallization, redistributed into zones of preferential permeability and stored in recrystallized P-rich albite, (2) the low-temperature destabilization of albite and lepidolite (host for radiogenic Sr produced by in-situ radioactive ~(87)Rb decay), during the Oligocene extensive period, provoked the consequent mixture of both Sr in variable proportions, resulting in the precipitation of goyazite. These results highlight the strong disturbance affecting the Rb-Sr granite system which can be visualized in secondary minerals, such as these low-temperature, micron-size Sr-rich phosphates.